Currently we have disk IOPS in monitoring (both read and write).
One of the metrics that play a role to decide whether to have a provisioned disk or not, at least with AWS hosting, is the disk bandwidth.
For instance with a large enough disk, like 2000GB, I have max 250MB/s bandwidth with an unprovisioned (gp2) disk (the maximum), but could go to 500MB/s with a provisioned (io1) disk of that size.